Definition
DPI vs PPI
PPI is the pixel density of an image file; DPI is the dot density a printer lays down. Print work asks for 300 PPI at final printed size — an image's pixel count only means something once you fix how large it will print.
The number that matters is pixels divided by printed inches. A 1200-pixel-wide image is 300 PPI across four inches and 150 PPI across eight, so a file can be simultaneously fine and unusable depending on placement. Nothing is gained by resampling upward: the pixels are not recovered, only interpolated, and the softness survives. Line art is the exception, wanting far higher densities because a stepped edge is more visible than a soft photograph.
Related tools & guides
A print-ready interior, checked
Upload the manuscript and download an exact-trim PDF with embedded fonts and the right gutter for its page count — or check the PDF you already have against KDP's rules first.